Lip skin differs markedly from the rest of the body. The stratum corneum is far thinner, there are almost no sebaceous glands and no sweat glands. This means lips cannot produce their own natural protective film — so they dry easily and are more vulnerable to environmental factors.
Moisture loss is faster on lips than facial skin. Cold air, wind, low humidity, frequent lip-licking and mouth breathing all increase it. Lip-licking gives momentary relief but worsens things as saliva evaporates — a common vicious cycle.
The basic approach is to provide a protective, moisture-supporting layer on the lip surface. Lips are also exposed to sun; products with sun protection help those spending time outdoors. See a physician for persistent cracking or discomfort.